This invention relates to asymmetric integrally-skinned flat sheet membranes and methods for making and using these membranes.
In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications of membrane gas separation have achieved commercial success, including N2 enrichment from air, carbon dioxide removal from natural gas and from enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex™ cellulose acetate spiral wound polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.
Polymers provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for gas separation. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore let smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through more slowly as compared to polymers with less stiff backbones. Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, and in chemical, thermal, and mechanical stability.
The membranes most commonly used in commercial gas and liquid separation applications are asymmetric polymeric membranes and have a thin nonporous selective skin layer that performs the separation. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as permeability or PA) and the selectivity (αA/B). The PA is the product of the gas flux and the selective skin layer thickness of the membrane, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
One of the components to be separated by a membrane must have a sufficiently high permeance at the preferred conditions or an extraordinarily large membrane surface area is required to allow separation of large amounts of gases or liquids. Permeance, measured in Gas Permeation Units (GPU, 1 GPU=10−6 cm3 (STP)/cm2 s (cm Hg)), is the pressure normalized flux and is equal to permeability divided by the skin layer thickness of the membrane. Commercially available gas separation polymer membranes, such as CA, polyimide, and polysulfone membranes formed by phase inversion and solvent exchange methods have an asymmetric integrally skinned membrane structure. Such membranes are characterized by a thin, dense, selectively semipermeable surface “skin” and a less dense void-containing (or porous), non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the “skin”. However, fabrication of defect-free high selectivity asymmetric integrally skinned polyimide membranes is difficult. The presence of nanopores or defects in the skin layer reduces the membrane selectivity. The high shrinkage of the polyimide membrane on cloth substrate during membrane casting and drying process results in unsuccessful fabrication of asymmetric integrally skinned polyimide flat sheet membranes using phase inversion technique.
US 2005/0268783 A1 disclosed chemically cross-linked polyimide hollow fiber membranes prepared from a monoesterified polymer followed by final cross-linking after hollow fiber formation.
U.S. Pat. No. 7,485,173 disclosed UV cross-linked mixed matrix membranes via UV radiation. The cross-linked mixed matrix membranes comprise microporous materials dispersed in the continuous UV cross-linked polymer matrix.
U.S. Pat. No. 8,016,124 disclosed a thin film composite membrane (TFC) comprising a blend of polyethersulfone and aromatic polyimide polymers. The TFC membrane has a layer of a blend of polyethersulfone and aromatic polyimide with a thickness from about 0.1 to about 3 microns.
U.S. Pat. No. 8,337,598 disclosed a TFC hollow fiber membrane with a core player and a sheath UV-crosslinked polymer layer.
The selective thin layer on the non-selective porous layer of a thin film composite (TFC) membrane can be delaminated easily from the non-selective porous layer, which will result in significantly decreased selectivity for gas separations. On the other hand, the integrally-skinned asymmetric membranes have a selective thin layer and a porous layer from the same membrane material and formed from the same membrane solution at about the same time. Therefore, the selective thin layer of an integrally-skinned asymmetric membrane cannot be delaminated easily from the non-selective porous layer.
The present invention discloses asymmetric integrally-skinned flat sheet membranes and methods for making and using these membranes.